The invention relates to a cooking surface for an induction heating cooker comprising a coil structure below the surface.
Flat and heat tolerant upper surfaces for cookers are well known, and their use in domestic kitchens is predominantly due to the ease with which one may move cooking utensils from one to the other cooking zone, the ease of cleaning, and the quick thermal response. However, most of these are made selfsupporting of a glass or vitroceramic material which have to be quite thick in order to withstand the dropping of cooking utensils. The differences in coefficient of expansion between the top plate of said glass or vitroceramic material and the supporting metal frame make the manufacture complex and the cleaning of the edges of the top plate is rather more difficult due to the need to allow expansion with respect to the surrounding frame.
The fairly thick glass or vitroceramic top plates which have become known in connection with radiative heating zones have been carried over into the area of induction heating, mainly because of the well-established manner of manufacture, provision with patterns and signs and because the material combines electrical insulation with strength. The hampering in energy transfer due to the spacing between the transmitting coil and the cooking utensil has been accepted in view of the other advantages.
In W094/05137 a cooking plate concept is described which utilises an induction heating arrangement comprising a ring-shaped magnetic core structure made of magnetically conductive concrete. This structure is embedded in further concrete material, and the top surface may be either cast upon the fairly solid base or be made of possibly porous alumina (aluminium oxide ceramic) which demonstrates a high thermal insulation and abrasion resistance. This latter material would be applied by plasma spraying. It is hence non-selfsupporting. However, this kind of top surface material, although thermally excellent, has shown certain disadvantages in practice. The inherent porosity makes the surface difficult to clean, although very strong detergents may be used, due to the chemical stability of alumina. The casting in place of a top layer of heat resistant concrete is efficient in the manufacturing process, but the finishing of such a surface and the sealing of the pores to permit cleaning are difficult tasks and the result may be destroyed by detergents. Hitherto it has been considered that a top layer should be as thin as possible for efficient power transport, in other words, the air gap should be minimized. Hence all endeavours in this field have worked towards this goal. The structure described above provides this possibility in a most efficient manner, but at the cost of a very massive construction.